1. Field of the Invention
The present invention relates to a device for controlling a power steering apparatus equipped on a vehicle such as an automobile, and more specifically, to such a device that controls torque and/or force, generated by a power steering apparatus, for assisting a driver's steering action and stabilizing the behavior of a vehicle.
2. Description of Prior Art
In a modern power steering system equipped on a vehicle, torque/force applied to steered wheels for assisting a steering action of a driver (steering assist torque) is controlled based upon parameters of running conditions of the vehicle, such as a vehicle speed. So far, in addition to the purpose of the reduction of torque required for rotating a steering wheel (steering torque), power steering systems and their control devices are variously designed so as to provide a driver with comfortableness and safety in driving a vehicle.
In some of power steering systems, torque controlled to be increased/decreased in the direction opposite to the rotational velocity of a steering wheel (the rate of changing the steering angle of steered wheels) is provided to the steering mechanism for steered wheels, as well as steering assist torque depending upon steering torque. This torque, often called as “damping control torque” or “steering converging torque”, suppresses the inertial rotation or angular momentum of the steering wheel, improving “steering convergency”, i.e. performance of controlling a steering angle against the overshooting and/or fluctuation of the steering system, or stabilizing the angular position of the steering system, to prevent a vehicle body from staggering left and right upon the rotating or releasing of the steering wheel. Examples of Electric Power Steering (EPS) systems, designed to generate such damping control torque, are seen in Japanese Patent No. 2568817 and Japanese Patent Laid-Open Publication No. 63-291769. In these publications, an input current value for an electrical motor moving a rack in a rack-pinion mechanism is determined based upon a sum of target steering assist torque value and target damping control torque value, calculated as functions of steering torque and steering angular velocity, respectively, and optimized with a vehicle velocity. Because of the improvement of the steering convergency through the application of damping control torque, the vehicle traveling direction is smoothly settled, thereby improving the straight-line stability in a running vehicle.
Excessive damping control torque, however, would deteriorate the smoothness in the rotating of the steering wheel. If steering torque required for rotating a steering wheel is excessively increased due to the exertion of the damping control torque, the driver would feel that her handle is too heavy when a driver intends to rotate a steering wheel relatively quickly and/or continuously. Thus, for the comfortableness in driving and/or steering a vehicle, the damping control torque should be controlled while taking into account driver's handling operation.
By the way, when a vehicle behavior is fallen into deteriorated conditions, i.e. oversteering (spinning) conditions or understeering (drifting-out) conditions, rather strong reciprocal yawing motions of a vehicle body would render steered wheels and a steering wheel instable, highly deteriorating the steering convergency. Although Vehicle Stability Control (VSC), if equipped on the vehicle, is executed to generate “anti-spin” or “anti-drift-out” yaw moment for suppressing the unwanted yawing motions, VSC would be less effective if the angular position of the steered wheels is liable to fluctuate. Rather, unwanted steering motions due to strong road reaction or panicky steering action of a driver would cause further deterioration of a vehicle behavior under oversteered or understeered conditions. In order to recover the steering convergency under such deteriorated conditions of a vehicle behavior, it is required to generate large torque on the steered wheels against their unwanted rotation, which torque, however, is excessive for damping control torque under normal running conditions in which the steering smoothness is required.
In conventional controlling strategies for damping control torque so far, the requirement of larger torque for recovering steering convergency upon deterioration of a vehicle behavior is less considered. Thus, control devices for power steering systems may be improved more appropriately in conjunction with control of torque for steering convergence, especially upon deterioration of a vehicle behavior. In such control devices, the steering smoothness should be also ensured under normal running conditions.